13:1–59 In chs. 12–15, the Levitical regulations—regulations for use by the priesthood from the tribe of Levi—concern ritual impurity related to the human body. The main topic of 13:1–14:57 is identified by the Hebrew word tsara'ath, commonly translated “leprosy.” The Hebrew term did not originally refer to leprosy in the modern sense of the English word, which usually designates a mycobacterial infection called Hansen’s disease (an affliction that likely did not exist in the Near East until the Hellenistic period, ca fourth century bc).

The Hebrew word tsara'ath is a general term covering a variety of infections that produce visible sores on the skin. From the symptoms described in vv. 1–44, tsara'ath likely includes the following skin diseases known to modern medicine: psoriasis, eczema, dermatitis, favus, scabies, vitiligo, and perhaps skin cancer. The use of tsara'ath for surface infections in textiles and building materials (vv. 47–59; 14:34–53) suggests an even broader range for the word. When tsara'ath applies to a person, a visible skin disease or infection of some kind is meant. When the term applies to inanimate objects, tsara'ath probably designates some kind of mold or mildew that can affect cloth and plaster.

Leviticus 13 explains how the priests should go about diagnosing an unclean skin disease. The basic process is the same regardless of the apparent cause of the infection (whether a burn, a boil, or other lesion). The affected person must appear before the priest for examination. In cases where the unclean nature of the infection is not immediately apparent, the priest would quarantine the person for seven days and inspect the infection again. To be declared clean, the marks would have to not spread and to have visibly healed over the course of 14 days of quarantine. If the condition had worsened on reinspection, then the person was pronounced unclean. The specific criteria used to diagnose the condition vary, but they generally include the color of the skin in the diseased area, the color of the hair in the diseased skin, and the size of the infected area.

13:9–17 This section addresses a new scenario—the possibility of chronic skin conditions (tsara'ath in Hebrew; see note on vv. 1–59). The diagnosis of this type of skin disease requires swelling and the presence of raw, open flesh in addition to the whiteness and white hair. If the open flesh heals completely, the person may be pronounced clean.

13:18–28 This passage addresses skin afflictions that develop following the healing of boils or burns. The diagnostic criteria in these cases are essentially the same as the basic condition described in vv. 1–8.

13:29–37 The skin irritations in this section relate to the skin of the scalp or face. The most likely condition behind what is described here is favus—a fungal infection, usually affecting the scalp, that produces yellowish, crusted lesions.

13:38–39 The widespread presence of dull white spots on the skin is not an indicator of impurity unless accompanied by other characteristic symptoms of skin ailments (tsara'ath in Hebrew; vv. 3, 10, 30). The condition described here could be the disorder known as vitiligo (or leukoderma). Vitiligo is characterized by portions of skin losing pigment.

13:40–44 This section begins by explicitly indicating that hair loss alone is not a cause of uncleanness (vv. 40–41). However, an unclean skin disease may break out on the bald scalp just like any other part of the body (vv. 43–44).

13:45–46 These two verses provide general guidelines for following the priest’s diagnosis. First, the person must show the outward signs of mourning—wearing torn clothes, having disheveled hair, and covering their mouths or upper lips (compare 10:6; 21:10–11; Ezek 24:17, 22). Second, they were to call out that they were unclean to warn others to stay away. Third, they had to live outside the camp.

13:47–59 This passage discusses tsara'ath visible in textiles and leather, which was considered analogous to disease on human skin (see note on Lev 13:1–59). The discoloration found in garments may have been caused by mold or mildew. If the problem was still present after a seven-day observation, the item would be burned. If the affected area did not grow larger, it was laundered.